Early saddles lacked stirrups and therefore had several shortcomings. For example, with a saddle with no stirrups a rider had to use his hands and grip with his legs to stay on the horse while moving at fast speeds as in hunting or battle. In addition, the rider had his full weight applied through his torso and buttocks onto the horse's back and with the weight being concentrated directly under seat made riding uncomfortable for rider and horse. As a horse galloped or trotted, the rider could not effectively separate his groin and buttocks from the horse's back, except by bouncing from the movement of horse, and this bouncing intensified the discomfort in contact areas.
The introduction of stirrups helped alleviate some of the aforementioned problems in that the rider could more easily brace himself by exerting force on the stirrups. Consequently, his hands were more free to hunt or use weapons or otherwise control the horse. Furthermore, he could stand on the stirrups to separate his torso from the bouncing of the horse's back in motion.
The introduction of the stirrup however, required a rigid frame or “saddle tree” for anchoring the stirrups straps. For example, in order to suspend the weight of a standing rider, the stirrups must be securely attached and supported. In addition the pressure resulting from this weight needed to be distributed over a large surface area to avoid soreness and tissue damage to the horse's back
This need for a rigid saddle tree to distribute the weight of rider, whether standing or sitting, made a close fit of the saddle and associated tree important for durability and health of the horse. When the tree did not fit evenly throughout its frame it would concentrate the pressure where it did contact the horse and this resulting concentration could inhibit or even stop blood flow to skin and muscles and bone under the saddle causing permanent damage to the back of the horse.
Attempts to fit a rigid tree to specific horses presented problems in that the same tree could not be used on different horses and because padding is traditionally added under a tree once it has been selected for use on a specific horse and padding changes the size and shape of a horse's back under the tree, the fit prior to addition of padding was not equal to the fit visualized before padding was applied. This led to development of trees with padding build in as seen on what are commonly termed “English” saddles.
In addition, not only do different breeds of horses vary in shape and size but animals within the same breed vary in size and shape, and an individual horse may vary from time to time, resulting from changes in age, diet and exercise.
Numerous attempts to create a saddle with adjustable fit have been made to adjust contact in limited ways. They use systems which either have flexible members which move away from pressure resulting in pressure concentrations at the remaining points of suspension or adjustable trees which require tools and do not make allowances for padding as traditionally applied under rigid tree saddles, the addition of which often causes the fit to become too tight and pressure areas to develop.